Electronic – Printf style debugging via SWD

I would be tempted to use alternative debugging techniques such as status LEDs or a serial output displaying progress messages, with both the code concerned and the required peripherals' power enabled by build flags. A lot of work has been done this way on systems where a breakpoint debugger was not available; used creatively it's extremely powerful and with the sub-minute build/flash/test cycles of today, pretty efficient.

Depending on how slow a clock rate you are dropping to, you might still be able to run a serial port at a standard baud rate (300 baud?, 75 baud with short abbreviations). If not, you could do logging via a synchronous protocol like SPI where baud rate can usually be allowed to vary widely, and use another micro-controller as an SPI slave which in turn re-outputs asynchronous data to your PC at a standard baud rate.

It may also be useful to accessorize your SWD debugger with automated control of the reset line; the VL discovery boards do this for the on-board target processor, but it doesn't seem to be part of the SWD connector spec. Also, if you have a firmware that goes into a low power mode quickly after boot, you can usually get the debugger going by holding down reset while commanding single-step mode, thus getting debug access before the mode switch can happen.

Reset pin is often not needed, because controller is possible to reset using debug registers.

Only one case when is necessary is connection under reset, this mean that if SWD signals are shared with other functionality on board (forexample SWDIO or SWDCLK is set as output), then only way to access MCU is to put MCU in to reset (at this moment are some debugging registers accessible over SWD) and set HALT/STOP program flag with debug registers over SWD, then leave reset pin and at this moment it is possible to flash MCU.